Nematic, smectic C, chiral nematic, chiral Smectic C and other liquid crystal devices are routinely used in display and other electro-optic applications. For many display applications luminance, grey scale and grey scale as a function of angle are important characteristics. Promising devices which potentially exhibit favorable luminance, grey scale and grey scale as a function of angle properties include vertically or homeotropically aligned cholesteric (VAC) devices, such as described in U.S. Pat. No. 5,466,358, and in-plane switching cells such as described in U.S. Pat. No. 5,477,358. For good switching rates and grey scale behavior as a function of viewing angle these devices require rapid, multi-domain control of the liquid crystal director.
The use of in-plane electric fields for the multi-domain control of the alignment of a liquid crystal has been described, for example, in U.S. Pat. No. 5,309,264. Refinements have been described in, for example, U.S. Pat. No. 5,777,711. None of these devices exhibits adequate and sufficiently rapid domain control when the liquid crystal is homeotropically aligned. Some of these patterns also require relatively precise positioning of the top and bottom electrodes relative to each other, which is difficult to accomplish and typically results in an appreciable loss of manufacturing throughput. Another approach involving in-plane electric fields involves two electrodes on at least one of the surfaces of a liquid crystal device as seen in U.S. Pat. No. 5,745,207. This results in large in-plane electric fields, but presents practical difficulties and requires numerous steps in connection with the manufacture of such devices.
This invention makes possible exceptionally good control over the alignment of liquid crystals in displays or other electro-optic devices. Control of the alignment of the liquid crystal is achieved by use of appropriately patterned electrodes. In particular, small scale patterns are created in the electrodes by non-conducting gaps having dimensions and shapes selected in accordance with the invention. These small scale patterns are contained largely or entirely within a single domain of the liquid crystal, often covering much of the area of the electrode. As a result, the instant invention allows for good control of the liquid crystal directors, which results in fast switching and good control over the number, size and location of liquid crystal domains. Moreover, the advantages of the invention can generally be achieved by relatively simple and inexpensive manufacturing techniques.
Accordingly, in one aspect of the invention there is provided a liquid crystal cell comprising first and second substrates spaced apart by a distance t and a liquid crystal material disposed therebetween. The first and second electrodes are disposed on said first and second substrates, respectively, and connected to a power supply. At least one of the first and second electrodes has at least one pixel defined by dimensions in a plane parallel to the plane of the substrate and further including at least one non-conducting gap therein, the non-conducting gap being a small scale gap having at least one dimension in the plane of the electrode that does not exceed about 2.5 times the distance t. When in a field-on condition in cooperation with the other of said first or second electrode, the pixel contains an electric field effective to produce at least one liquid crystal domain having dimensions in a plane parallel to the plane of the substrates within which the liquid crystal molecules have an azimuthal orientation with predominantly the same sign and direction, and wherein at least a portion of the small scale gap is disposed within the domain at least about 1 times, and preferably at least 2.0 times, the distance t from the boundaries of the domain. Preferably, the cell includes a plurality of said small scale gaps within said at least one pixel.
In one embodiment, when in said field-on condition, the liquid crystal material exhibits a plurality of domains within the pixel, each said domain being adjacent at least one said small scale gap, or in another embodiment, each said domain is adjacent a plurality of said small scale gaps. In yet another aspect of the invention, both said first and second electrodes include at least one pixel containing at least one said small scale gap or, in another embodiment, a plurality of said small scale gaps. In another embodiment where both the first and second electrodes include at least one pixel, in a field-on condition, the liquid crystal material exhibits a plurality of domains within each pixel, each domain being adjacent at least one small scale gap or a plurality of said small scale gaps. In many embodiments of the invention, the pixels on the first and second substrates are substantially adjacent and coextensive.
In still other embodiments, at least a portion of a boundary of the domain is substantially linear, and the boundary is substantially adjacent and colinear with at least one of a non-conducting gap or portion of a non-conducting gap, or a difference in a location of edges of the electrodes on opposing substrates at an edge of the pixel, and a plurality of said small scale gaps is disposed at an angle thereto, or extend at an angle therefrom. In many embodiments, the small scale gaps are substantially rectangular and parallel to one another. In other embodiments, all the domain boundaries are substantially linear and disposed substantially adjacent and colinear with a non-conducting gap or a portion of a non-conducting gap, or with a difference in a location of edges of the electrodes on opposing substrates at an edge of the pixel, with a plurality of said small scale gaps disposed at an angle thereto or extending at an angle therefrom. Preferably, the small scale gaps are substantially rectangular and parallel. In some preferred embodiments, both said first and second substrates include at least one such pixel such that the pixels are substantially adjacent and coextensive. In other embodiments, when each pixel includes a plurality of said small scale gaps, the small scale gaps are substantially rectangular and parallel to one another, and the substantially parallel gaps on opposing said substrates are rotated relative to each other by an angle of from about 10xc2x0 more to about 30xc2x0 less than an angle by which the azimuthal orientation of said liquid crystal rotates as a consequence of its natural pitch on passing through the cell. Preferably, the liquid crystal material is selected from a nematic liquid crystal or a chiral nematic liquid crystal having negative dielectric anisotropy, and at least one said substrate is treated to align the liquid crystal.
It is another aspect of the invention wherein at least about 60% of the area within at least one pixel is within about 1.5 times the distance t, and preferably 0.7 times the distance t from an edge of a conducting portion of the electrode. In many preferred embodiments at least about 80%, and more preferably still 90%, of the area of the pixels will be so designed. In one such embodiment, when in a field-on condition, the liquid crystal material exhibits a plurality of domains within the pixel, each domain being adjacent a plurality of said small scale gaps wherein at least about 60% of the area within said at least one pixel is within about 1.5 times the distance t from an edge of a conducting portion of said electrode.
Preferably, the small scale gaps form a pattern within each liquid crystal domain which transforms according to a two-dimensional space group selected from the group consisting of Pg, Cm or Pl.
It is yet another aspect of the invention to provide a liquid crystal cell wherein the pixel includes a plurality of small scale non-conducting gaps having a length and a width in the plane of said electrode wherein said width does not exceed about 2.5 times the distance t and wherein at least about 60% of the area within said at least one pixel is within about 1.5 times the distance t from an edge of a conducting portion of said electrode such that, when in a field-on condition in cooperation with the other of said first or second electrode, the pixel contains an electric field effective to control the of the liquid crystal directors of adjacent liquid crystals within said pixel. Preferably, at least about 90% of said area within said pixel is within about 1.5 times the distance t from an edge of a conducting portion of said electrode. In a preferred embodiment, the gaps have a length greater than said width and are generally parallel. Still more preferably, the gaps are substantially rectangular and parallel to one another. In other embodiments, at least about 60%, preferably at least about 80%, and still more preferably at least about 90% of the area within said at least one pixel is within about 0.7 times the distance t from an edge of a conducting portion of said electrode. In some embodiments, the pixel will include at least one non-conducting gap or portion of a non-conducting gap substantially adjacent and colinear with at least a portion of a boundary of said domain, and a plurality of said small scale gaps disposed at an angle thereto or extend at an angle therefrom. In some embodiments, said small. scale gaps being substantially rectangular and parallel to one another. In many embodiments of the invention, both said first and second substrates include at least one pixel such that the pixels are substantially adjacent and coextensive. In some embodiments, the substantially parallel gaps on opposing substrates are rotated relative to each other by an angle from about 10xc2x0 more to about 30xc2x0 less than an angle by which the azimuthal orientation of said liquid crystal rotates as a consequence of its natural pitch on passing through the cell.
It is yet another aspect of the invention, in the liquid crystal cell comprising first and second substrates spaced apart by a distance t; a liquid crystal material disposed therebetween; first and second electrodes disposed on said first and second substrates, respectively; at least one of the first and second electrodes including at least one pixel defined by dimensions in a plane parallel to the plane of said substrate, and further including a plurality of non-conducting gaps therein, to provide the non-conducting gaps so as to have dimensions in the plane of the electrode and be arranged such that the gaps form a pattern within the pixel whereby, when in a field-on condition in cooperation with the other of said first or second electrode, said electrodes give rise to a plurality of spatial harmonics of an electrical potential within a liquid crystal domain. The harmonics include at least one triplet of harmonics. The spatial harmonics further have a period less than the largest dimension of said liquid crystal domain. At least one triplet of the plurality of spatial harmonics have wavevectors that, when added together, equal zero and further: i) one of said triplet of harmonics by itself, and the remaining two harmonics acting together, result in spatial variations in the magnitude of a vector D, the direction of a vector D or both, such that these variations have the same sign when D has one sign, and have a different sign when D has another sign; or, ii) complex amplitudes of at least one of the triplets of harmonics, evaluated at one of the substrates, when multiplied together have a large imaginary part; or, iii) complex amplitudes of at least one of the triplets of harmonics evaluated at one of the substrates has a range of magnitudes of said wavevectors, and said magnitudes, when multiplied together and also multiplied by the sum over permutations of the wavevectors of one wavevector times the dot product of the other two wavevectors and summed or integrated over this range of wavevectors results in a sum or integral with a large imaginary part, whereby the electrodes form one or more liquid crystal domains having dimensions in a plane parallel to the plane of said substrates within which said liquid crystal molecules have an azimuthal orientation with predominantly the same sign and direction.
In one aspect of the invention, each said substrate includes at least one such pixel. Preferably, said pixels are substantially adjacent and coextensive. In another aspect of the invention, one substrate includes a plurality of such pixels. It is another aspect still for both substrates to include a plurality of such pixels and, preferably, each such pixel is substantially adjacent and coextensive with a corresponding pixel on the opposing substrate. In some embodiments, at least 60% of a pixel""s area is within about 1.5 times the distance t from an edge of a conducting portion of an electrode. In other embodiments, each substrate includes at least one pixel having at least 80% of its area within about 0.7 times the distance t from an edge of a conducting portion of the electrode. Preferably, the pattern of small scale gaps within each liquid crystal domain transforms according to a two-dimensional space group selected from the group consisting of Pg, Cm or Pl. In another aspect of the invention, within at least one liquid crystal domain, said pattern of small scale gaps on one substrate can be obtained from the pattern on the other substrate by rotating the pattern by 180xc2x0 around an axis passing through the center of the cell perpendicular to the substrate normal. In some embodiments, said rotation axis makes an angle to a direction perpendicular to a longitudinal direction of the gaps between about 5xc2x0 less than to about 15xc2x0 more than half the angle an azimuthal orientation of the liquid crystal will rotate on passing through the cell. Preferably, the substrate including said at least one pixel is further treated to promote alignment of said liquid crystal. In some embodiments, both said substrates are treated to promote alignment of said liquid crystal. In still further embodiments, the liquid crystal material is selected from a nematic liquid crystal or a chiral nematic liquid crystal having negative dielectric anisotropy and one or both substrates is treated to promote homeotropic alignment of the liquid crystal.
It is still another aspect of the invention to provide a method. More particularly, in a liquid crystal cell comprising first and second substrates spaced apart by a distance t and a liquid crystal material disposed therebetween, and first and second electrodes disposed on the first and second substrates, respectively, and connected to a power supply, the invention provides a method of controlling the sign and direction of an azimuthal orientation of said liquid crystal. The method comprises disposing non-conducting gaps in at least one of the electrodes having dimensions in the plane of the electrode and being arranged such that the gaps form a pattern in the electrode effective to, in a field-on condition in cooperation with the other electrode, give rise to a plurality of spatial harmonics of an electrical potential within a liquid crystal domain, which harmonics include at least one triplet of harmonics, said spatial harmonics having a period less than the largest dimension of said liquid crystal domain, at least one triplet of said plurality of spatial harmonics having wavevectors that, when added together, equal zero and further wherein i) one of said triplet of harmonics by itself, and the remaining two harmonics acting together, result in spatial variations in the magnitude of a vector D, the direction of a vector D or both, such that these variations have the same sign when D has one sign, and have a different sign when D has the another sign; or, ii) complex amplitudes of at least one of said triplets of harmonics, evaluated at one of said substrates, when multiplied together have a large imaginary part; or, iii) complex amplitudes of at least one of said triplets of harmonics evaluated at one of said substrates has a range of magnitudes of said wavevectors, and said magnitudes, when multiplied together and also multiplied by the sum over permutations of said wavevectors of one wavevector times the dot product of the other two wavevectors and summed or integrated over this range of wavevectors results in a sum or integral with a large imaginary part.
In one embodiment, the method comprises disposing the non-conducting gaps so as to produce a plurality of said domains. In another embodiment, the method comprises disposing the non-conducting gaps so as to produce at least one pixel having dimensions in a plane parallel to the plane of said substrates within which at least about 60%, preferably at least about 80%, and more preferably still at least about 90%, of the area of the pixel is within about 1.5 times the distance t, and preferably within about 0.7 times the distance t from an edge of a conducting portion of said electrode. In some aspects of the invention, the non-conducting gaps are disposed wherein at least a portion of a boundary of a domain is substantially linear, and the boundary is substantially adjacent and colinear with at least one of a non-conducting gap or portion of a non-conducting gap, or a difference in a location of edges of the electrodes on opposing substrates at an edge of said pixel, and further comprising a plurality of small scale gaps disposed at an angle thereto. Preferably, the method comprises disposing the non-conducting gaps in a pattern which transforms according to a two-dimensional space group selected from the group consisting of Pg, Cm or Pl. In one aspect of the invention, the method comprises providing at least a portion of the non-conducting gaps so as to create small scale gaps having at least one dimension in the plane of the electrode that does not exceed about 2.5 times the distance t, and such that at least a portion of the small scale gaps is disposed within said domain at least about 1 times the distance t from the boundaries of said domain. Preferably, the gaps are disposed so as to produce a plurality of liquid crystal domains.
It will be apparent to one of ordinary skill in the art that the foregoing principles can be useful in numerous liquid crystal display or other electro-optic applications. The electrode patterns described herein can be used in conjunction with a number of cells wherein the liquid crystal director has more than two possible orientations in a field-on condition. These include, but are not limited to, the homeotropically aligned liquid crystal cell, described in detail hereinafter, a twisted nematic cell with planar alignment wherein the alignment twists an angle of 90xc2x0 on passing through the cell, a hybrid cell with negative dielectric anisotropy wherein the orientation of the liquid crystal director near the homeotropically aligned surface is controlled by the patterned electrode of this invention. Such electrodes may also be useful for cells filled with smectic C, chiral smectic C or other tilted chiral or achiral smectics, such as smectic Cxcex3 wherein the tilt of the director in subsequent layers is more complex than that realized in a smectic C. The patterned electrodes of the invention may particularly be useful in cells in which the layer normal of the smectic liquid crystal material is parallel to the normal of the electrodes or substrates. They may also be useful in dynamically determining the orientation of a nematic or other director as it relaxes from one state to another.
One preferred application of the present invention is control of the alignment of the director in any liquid crystal cell that has homeotropic alignment. An example of such a liquid crystal cell is one in which both sides of the cell have been treated to promote homeotropic alignment and which employs an achiral nematic liquid crystal. Another is a so-called xe2x80x9chybridxe2x80x9d cell, as noted above, in which one electrode has been rubbed, either uniformly or patterned, or otherwise treated to promote planar alignment, and the other electrode is treated to promote homeotropic alignment, and patterned in accordance with the present invention. When such a device employs an active matrix, the invention advantageously obviates the need to rub the substrate containing the active matrix. Appropriate patterning of the unrubbed substrate in accordance with the invention will result in an advantageous increase in the number of domains in the liquid crystal cell.
In a particularly preferred application the present invention is used to control the director in a nematic or cholesteric liquid crystal cell, such as that described in U.S. Pat. No. 5,477,358, incorporated herein by reference. This cell consists of a chiral nematic liquid crystal homeotropically aligned between two electrodes so that the director of the liquid crystal is parallel to the z-axis, i.e., the direction normal to the surface of the electrode. The liquid crystal has negative dielectric anisotropy so that when a sufficiently large voltage is applied the liquid crystal director in the cell will tilt. With appropriate compensators this allows for a very dark xe2x80x9cblackxe2x80x9d state, and other desirable viewing behavior. Application of this invention to such a device will control the direction of the tilt in the xy-plane, enabling the relatively simple production of high performance, multi-domain devices with large viewing angles and grey scale.
Still further, the present invention can produce multidomain patterns in a nematic twist cell. Such a cell consists of two surfaces rubbed in such a way as to provide alignment in the plane of the surface. The surface rubbing is patterned such that the director rotates approximately 90xc2x0 moving from one surface to the other, in either a clockwise or counterclockwise direction. In such cells, the addition of chiral dopants or surface treatments to provide a pre-tilt in the liquid crystal have been used to impart a preference between these two possible directions of rotation. Except for a patterned pre-tilt, which requires complicated manufacturing steps, these techniques do not allow for the rotation to be in one direction in some parts of the cell and in the opposite direction in other parts of the cell. By contrast, the patterned electrodes of the present invention allow for control of the direction of twist throughout the cell without additional process steps.
Finally, it is also possible to use the present invention on an electrode that has been treated for planar or nearly planar alignment. If the anchoring strength is small, then the present invention will allow reorientation of the nematic director when the electric field is applied to obtain the same results as an in-plane switching electrode without the significantly more complicated manufacturing requirements associated therewith.
These and other advantages and a fuller understanding of the invention will be had from the following detailed description of the preferred embodiments and accompanying drawings.